THE PYCNOGONIDS. 49 



PART III.— THE STRUCTURE AND DEVELOPMENT 

 OF THE EYES OF PYCNOGONIDS. 



In reaching a decision as to the systematic position of the 

 Pycnogonids, I hoped to test the conclusions I had reached from 

 a study of the embryology by taking another point of view. 

 Fortunately a most interesting field opened in a consideration of 

 the structure and development of the eyes. While enjoying the 

 opportunities of the Marine Biological Laboratory I was enabled 

 to procure and study an abundance of fresh material. The 

 advantages of using such material is now so well known as to 

 need only mention, and I am certain that I could not have 

 obtained nearly so satisfactory results from preserved animals. 



The results obtained from a study of the eyes have added, I 

 believe, very much to the solution of the problem I had before 

 me. Hoek and Dohrn give the only figures of the Pycnogonid 

 eyes that I have found, but the results are confessedly incom- 

 plete, due in the one case to poor preservation of material, and 

 in the other to the difficulties of technique. Neither of these 

 accounts will be of any service to us in our study of the eyes, 

 and may be dismissed here. The subject will be treated in the 

 following order : 



1. The Anatomy of the Adult Eyes. 



2. The Histology of the Adult Eyes. 



3. The Development of the Eyes. 



4. Comparison with the Arachnid Simple Eyes. 



5. Inversion of the Pycnogonid Eye. 



6. General Considerations of the Arachnid Eye. 



1. The Anatomy of the Adult Eyes. 



The Sea-Spiders have in all four simple eyes, situated in the 

 middle of the first (compound) segment of the body, on a raised, 

 cupola-like projection. In some of the species the eyes are cer- 

 tainly degenerate, but in none are all traces of the eyes lost. 

 The position of the eyes on the body can be best shown by 

 reference to the figures. In Fig. XXII we see the dorsal surface 

 of Tanystylum, and on this, in the so-called first segment, raised 



